1. Field of the Invention
This invention relates to methods and compositions for removing copper and copper oxides from surfaces, and more particularly, to methods and compositions for dissolving copper and copper oxide deposits whereby alloy surfaces contacted by the compositions are not adversely affected thereby.
2. Description of the Prior Art
In process equipment such as steam boilers, feed water heaters, heat exchangers and pressure vessels in which water is circulated, water insoluble salts commonly deposit on the interior metal surfaces of such equipment. The formation or deposition of scale deposits can markedly reduce the heat transfer and/or the capacity of flow passages in the equipment.
In process equipment which includes or is associated with components constructed of copper alloys, the scale produced on internal surfaces is frequently found to contain copper and copper oxides. A variety of methods and compositions have been developed and used heretofore for removing such copper and copper oxide deposits. For example, one method which has been employed for removing copper and copper oxide scale deposits from ferrous metal surfaces is to initially contact the surfaces with an ammoniacal oxidant wash, such as an ammoniacal solution containing ammonium persulfate or sodium bromate to remove part of the copper deposits followed by contacting the surfaces with a cleaning solution containing an acid plus a copper complexing material. The copper complexing material functions to tie up the remaining copper so that it is dissolved and held in the cleaning solution. While such multi-stage cleaning procedures have been used successfully, they are generally expensive to carry out.
Another method of removing copper and copper oxides from ferrous metal surfaces is disclosed in U.S. Pat. No. 4,452,643 issued June 5, 1984. That method comprises contacting copper and copper oxide deposits with an aqueous composition having a pH of from about 3 to about 6 comprised of an oxidizing agent, preferably hydrogen peroxide, a compound selected from the group consisting of oxalic acid and the alkali metal and ammonium salts of oxalic acid and an ingredient selected from the group consisting of citric acid, polyaminocarboxylic acid, the ammonium and alkali metal salts of citric acid and polyaminocarboxylic acids and mixtures thereof.
While the above described methods have been utilized successfully in removing copper and copper oxide deposits from ferrous metal surfaces, in some process equipment such as nuclear steam generators, some of the internal surfaces are formed of alloys such as austenitic steel alloys, alloys of nickel, iron and chromium, alloys of nickel and chromium and other similar alloys. It has been found that when these alloys are contacted with copper and copper oxide scale removal cleaning solutions containing strong oxidizing agents, e.g., sodium bromate and ammonium persulfate, the alloys can be adversely affected such as by the occurrence of stress corrosion cracking. That is, sodium bromate is believed to cause cracking of austenitic stainless steel and other alloys due to the presence of bromide in the spent solution. Ammonium persulfate is believed to cause intergranular attack of nickel-chromium alloys due to the presence of sulfur. In addition, ammonium persulfate is unstable at temperatures above about 110.degree. F., sodium bromate contains a form of the element bromine which can cause personnel hazards and hydrogen peroxide is unstable in some cleaning solutions.
By the present invention an improved copper and copper oxide cleaning solution is provided which does not adversely affect surfaces of formed of austenitic, nickel-chromium and other similar alloys when contacted therewith. In addition, the cleaning solution, and particularly the oxidizing agent utilized therein, is more economical than heretofore used solutions and oxidizing agents.